The present invention relates to a fuel injection device for a ramjet operating at a high Mach number, for example of about 12 to 15.
It is known that ramjets are particularly advantageous for propelling hypersonic aircraft (missiles, airplanes, etc.) since they allow operation over a wide range of Mach numbers, for example from 2 to 15, and they have a low specific fuel consumption. Depending on the application peculiar to an aircraft, and possibly during the flight phase of the latter, the fuel used may be a liquid hydrocarbon, such as kerosene for example, or a gas, such as hydrogen or methane, for example.
It is also known that a ramjet includes, on the one hand, at least one oxidizer inlet, usually consisting of an air duct or an air intake, which directs an oxidizer flow (i.e. air) towards a combustion chamber and, on the other hand, at least one injection device which enables the fuel to be injected into said oxidizer flow so as to obtain a flux of oxidizer/fuel mixture which is ignited in said combustion chamber.
In ramjets designed to operate at a relatively low Mach number (for example up to Mach 2), such a fuel injection device may consist of a number of elementary injectors arranged in the internal wall of the ramjet, on the periphery of the oxidizer flow.
However, for operation at high Mach numbers, when combustion in the ramjet takes place in a supersonic or hypersonic flow, the fuel can no longer be injected just at the internal wall of the ramjet. This is because, in this case, the penetration of the jets of fuel into the oxidizer flow is too low for it to be possible to obtain good mixing of the oxidizer and the fuel within said flow, so combustion is poor or even impossible. Of course, such a drawback becomes more acute the greater the transverse dimensions of the oxidizer flow.
Thus, in order to remedy this situation, injection devices in the form of rails have already been provided, these devices having a number of elementary injectors distributed over their length and arranged in said oxidizer flow, transversely to the latter, while the ends of said rails are fastened to opposite walls of said ramjet. Such an injection device is generally called an "injection stub" and it is used either alone or in combination with fuel injection in the wall.
By using injection stubs, it is thus possible to obtain a satisfactory oxidizer/fuel mixture over the entire cross section of the oxidizer flow. More generally, the injection stubs installed in a hypersonic ramjet make it possible:
to feed fuel into the entire oxidizer flow, despite the low penetration of the jets of fuel into an oxidizer flow at hypersonic velocities; PA1 to increase the proportion of fuel in the oxidizer/fuel mixture; PA1 to assist in the ignition of the oxidizer/fuel mixture and to stabilize the flame; and PA1 to help compress the oxidizer flow, by reducing the rate of flow of fuel taken up by the ramjet. PA1 in that said nose is formed, at least near its leading edge, by a thermally conductive thin wall having at least approximately the shape of a dihedron, the angle of said dihedron being at most equal to 15.degree. and the radius of the leading edge of said nose being at most equal to 2 mm; and PA1 in that said stub includes: PA1 the thickness of said thin wall of the nose to be at most equal to 2 mm; PA1 the constituent material of said thin wall to have a thermal conductivity of 70 W/(m.K); and PA1 the coolant to be a low-temperature gas, for example hydrogen at a temperature of 100 K to 300 K. PA1 if hydrogen is used at a different temperature, it will be possible, all other things being equal, to maintain the wall temperature by modifying said flow rate; PA1 if the constituent material of the thin wall withstands a temperature greater than 1500.degree. C., for example 2000.degree. C., this constituent material may have a thermal conductivity of less than 70 W/(m.K) or the cooling may be less powerful; PA1 etc. PA1 in that it includes: PA1 in that said nose piece is joined to said body, in a sealed manner, by said thin faces; and PA1 in that, in the position in which said nose piece is joined to said body, said sealed chamber is formed in the concavity of said nose piece, between the latter and said body. PA1 at least one longitudinal groove made on the surface of said body and closed off by one face of said nose piece; and PA1 an array of transverse surface grooves bringing said sealed chamber into communication with said longitudinal groove and also closed off by said nose piece.
Such injection stubs, which are exposed to the action of the oxidizer flow, therefore each behave, from the aerodynamic standpoint, as an airfoil embedded at its ends in two opposite walls of the ramjet. In addition, on that side of their nose which receives the oxidizer flow, said injection stubs must have a leading edge with a small radius for limiting the pressure drops which would restrict the propulsive performance of the ramjet and could even lead to choking of the oxidizer flow, which can remain hypersonic in the combustion chamber only if the velocity of the oxidizer upstream is sufficiently high.
However, the heat-up of said nose produced by the hypersonic oxidizer flow is approximately inversely proportional to the square root of the radius of the leading edge of said nose. Thus, a nose with a small leading-edge radius heats up a great deal. Moreover, it will be noted that, since said injection stubs are arranged inside the ramjet, it is impossible to cool them by radiation with the air through which the aircraft propelled by said ramjet is flying. Such a nose is therefore exposed to very high temperatures: about 5000 K by an aircraft flying at Mach 12 at an altitude of about 30 km. It is therefore necessary to construct the injection stubs from materials such as ceramics, the radius of said leading edge being about 3 to 5 mm. However, in view of the current processes for producing ceramic components, it may readily be imagined that the precision manufacture of ceramic injection stubs, is necessarily lengthy and expensive.